Recently, a secondary battery such as a lithium ion secondary battery and a nonaqueous electrolyte secondary battery has been actively researched and developed as a high energy-density battery. The secondary battery is anticipated for use as a power source for hybrid vehicles, electric vehicles, an uninterruptible power supply for base stations for portable telephones, or the like. Therefore, the secondary battery is demanded to, in addition to having a high energy density, be excellent in other performances such as rapid charge-and-discharge performances and long-term reliability, as well. For example, not only is the charging time remarkably shortened in a secondary battery capable of rapid charge and discharge, but the battery is also capable of improving motive performances in hybrid automobiles, and the like, and efficient recovery of regenerative energy of motive force.
In order to enable rapid charge-and-discharge, electrons and lithium ions must be able to migrate rapidly between the positive electrode and the negative electrode. However, when a battery using a carbon-based negative electrode is repeatedly subjected to rapid charge-and-discharge, precipitation of dendrite of metallic lithium on the electrode may sometimes occur, raising concern of heat generation or ignition due to internal short circuits.
In light of this, a battery using a metal composite oxide in a negative electrode in place of a carbonaceous material has been developed. In particular, in a battery using an oxide of titanium in the negative electrode, rapid charge-and-discharge can be stably performed. Such a battery also has a longer life than in the case of using a carbon-based negative electrode.
The potential of Li4Ti5O12, which is a typical titanium oxide as a negative electrode active material, is about 1.5 V (vs. Li/Li+) with respect to the oxidation-reduction potential of lithium. On the other hand, the potential of a composite oxide Li2Na2Ti6O14 is as low as about 1.3 V (vs. Li/Li+) on the average. Thus, a high voltage is expected when Li2Na2Ti6O14 is combined with a positive electrode. Element substitution may be performed to obtain Li2Na2−xTi6−xMxO14 (M=Fe, Co, Mn, Al, Zr, Sn, V, Nb, Ta, or Mo), thereby forming a vacancy at Na sites in the crystal structure. The number of Li insertion/extraction sites is thereby increased to increase the capacity of the composite oxide.